1. Field of the Invention
This invention relates to novel polyurethane polymers and is more particularly concerned with polyurethane polymers and their preparation from novel isocyanate prepolymer blends.
2. Description of the Prior Art
Polyurethane polymers and elastomers, prepared from high molecular weight polyols and organic polyisocyanates and extended with low molecular weight extenders, particularly diols and diamines, have been known and utilized for a long time.
In the course of the development of this art the use of liquefied isocyanate components has become of increasing importance with the advent of high pressure reactant mixing in the reaction injection molding (RIM) technique. In this connection, methylenebis(phenyl isocyanate), [MDI], which is solid at room temperature, has received the most attention because this isocyanate offers the optimum combination of good reactivity and yields products having good physical properties.
Typical of a non-urethane containing liquefied MDI product is the one made in accordance with U.S. Pat. No. 3,384,653 which is a liquid at ambient temperatures because of its content of uretoneimine. However, a much greater variety of liquefied MDI products have been disclosed which are in stable liquid form because a minor portion of the isocyanate groups has been reacted with active hydrogen containing components, particularly organic polyols. For example, see U.S. Pat. Nos. 3,394,164; 3,394,165; 3,644,457; 3,883,571; 4,115,429; 4,118,411; 4,229,347; and British Patents 1,369,334; 1,377,676; 1,430,455; 1,545,003; and 1,557,767.
Also disclosed in the prior art as liquefied MDI products are those derived from blends of both of the above types (U.S. Pat. No. 4,031,026).
One of the very important properties of molded polyurethanes is the green strength which they possess upon sample demold (which will be discussed in detail below). It has been already disclosed (U.S. Pat. No. 4,065,410) that a combination of liquefied MDI products with polyols and extender mixtures containing ethylene glycol and at least one other polyol having a molecular weight below 1800 and a blowing agent provide molded polyurethanes characterized by improved green strength. Further, the use of particular catalysts has been noted in U.S. Pat. No. 4,239,857 for enhancing green strength.
We have now discovered a novel class of blends comprising liquefied MDI products blended with liquid MDI prepolymers prepared from particular mixtures of high and low molecular weight polyols which blends when used in the preparation of molded polyurethanes give rise to an unexpected increase in sample green strength. The attainment of increased green strength does not detract from the other polymer physical properties, and, surprisingly, in some cases the polyurethanes have superior physical properties over prior art materials.
The improvement in sample green strength leads, additionally, to lower molding temperatures and molded parts having improved properties over prior art polyurethanes when compared in both the green state and cured state.